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A calibrated dual-port acoustic Vector Network Analyser

The Vector Network Analyzer (VNA) is a workhorse in electrical engineering, and has been refined over the last fifty years to offer many advantages. This technology is continually expanding and branching into other fields including optics. This thesis presents the design and calibration of an Acoustic Vector Network Analyzer (AVNA), which represents further adaptation of this technology into other engineering fields. This goal requires several developments; a repeatable acoustic waveguide flange/joint, a calibration method(s), calibration standards, and calibration verification. The hardware that has been developed is used as an accessory to an existing commercial VNA mainframe (Agilent-4395A) and a retrofitted test-set (HP87511A) that now hosts connectivity for our ``measurement heads'' containing the acoustic directional couplers. Directional couplers are the fundamental technology that allows for Vector Network Analysis, the acoustic variant has an operational range of a little more than an octave and sufficient directionality for calibration. The coupler is a branch waveguide coupler and to enable reliable and repeatable connection has been equipped with a flange system. This flange system has alignment pins and O-rings to ensure proper sealing and repeatability. Several materials were used with additive manufacturing techniques to create couplers and waveguide sections, each with its own advantages. Titanium (Ti64) offered the best usability and reliability, while plastic offered better repeatability. Calibration was a major advance in the VNA and one that popularized the instrument. The calibration's performance is limited by the repeatability of flanged joints. This is because several measurements are required to produce a linear system of equations, therefore this system contains within it the variation of the flange joints. Developing a calibration method is the crux in realizing the aim of an Acoustic VNA. This thesis presents a set of five measurements and two competing algorithms that can be used to implement calibration. The analytical and numerical methods presented both have distinct advantages and disadvantages. The five measurements are performed with proposed calibration standards. A ``Thru'', ``Reflect'' and ``Match'' standard are required. The ``Match'' standard is provided by a sliding load, the sliding load has been previously used as part of an acoustic calibration. To verify the operation of the calibrated instrument verification measurement was performed using a passive, asymmetrical, reciprocal device (PARD).
Type of thesis
The University of Waikato
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